1. Field of the Invention
The present invention relates in general to regenerative thermal oxidation ("RTO") equipment and methods for operating same, and in particular to improvements in RTO equipment, operating cycles and hydraulic controls designed to stabilize air flow rates through the RTO equipment and to provide for more efficient self-cleaning cycles.
2. Description of the Related Art
As world-wide environmental awareness heightens, industry is being confronted with increased governmental requirements for treating air laden with contaminants from industrial processes. Regulations governing the emissions of volatile organic compounds are no exception. To remain competitive in the global marketplace, companies having manufacturing facilities will need to find cost-effective solutions for emission controls. Such emissions are often associated with manufacturing plants for high-speed printing, food processing, metal processing, painting, can manufacturing and the like.
It is well-known that noxious fumes containing volatile organic compounds (VOCs), including hydrocarbons or undesirable odors, can be destroyed at temperatures of about 1500.degree. Fahrenheit or higher. The fumes, specifically the contaminants in the air, are converted at such temperatures into harmless water vapor and carbon dioxide. Generally, thermal incineration systems designed to accomplish this conversion can usually be categorized as a recuperative design or a regenerative design. Briefly, recuperative systems utilize plate or tubular-type heat exchangers to obtain the required pre-heat temperatures necessary to purify contaminated air. Regenerative systems utilize high-density heat storage media to transfer heat directly to the incoming process air stream. The major advantage of regenerative systems is the ability to achieve primary heat recovery efficiencies in excess of 95 percent while eliminating on the order of 95 to 98 percent (or more) of the combustible contaminants.
Generally, regenerative thermal oxidizers include two or more fixed bed regenerative heat exchangers connected to a common combustion or incineration chamber. Contaminant-laden air is directed through one of the heat exchangers where it is pre-heated. Thereafter, it is directed to the combustion chamber where the contaminants are burned, that is completely oxidized. Because a mixture of combustion air and hydrocarbon fuel is added in the combustion chamber, hot combustion gases are produced, and these are then exhausted through another of the heat exchangers where they give up a major portion of their heat content before being discharged to the atmosphere through an exhaust fan. Periodically, flow through the system is reversed. The incoming contaminant-laden air is then heated by the heat exchanger which was previously heated by the exiting combustion gases, and the heat exchanger previously cooled by the incoming air is re-heated by the exiting combustion gases.
The flow reversals through the chambers are accomplished by having separate inlet and outlet flow structures connected to each chamber. These flow structures include main ducts and branch ducts in which large butterfly valves are located. The valves are opened and closed by valve operator units that are electrically, mechanically or hydraulically driven. The exhaust fan is typically a centrifugal fan, and continuously runs in one direction, even while the flow reversals from the heat exchange chambers are taking place.
During the periodic flow reversals, there is an opportunity for small amounts of unprocessed contaminated air to be transferred from the inlet side of the heat exchange chambers directly to the outlet side and exhaust fan. The undesired transfer occurs when contaminated gases that have already entered a heat exchange chamber, but have not yet reached the combustion chamber, are drawn out as the chamber is switched over so as to be connected to the outlet flow structure rather than the inlet flow structure. To avoid this problem, a flush flow structure, including main and branch flushing ducts and flushing valves located in the branch ducts, have been added to RTO equipment. The flush flow structure and operating of the flushing valves further complicate the task of attempting to finely regulate and stabilize the flow of process air drawn from plant processing equipment into an RTO.
While the approaches of prior regenerative thermal oxidizers have been successful, they have not been entirely satisfactory. For example, prior systems have not been able to finely control mass flow through the system, on account of the periodic flow reversals required through the heat exchange chambers. To assist in achieving even flow rates, very expensive variable speed electric motor drive systems have been used to drive the large exhaust fan at some constant speed selected from a range of possible speeds. This tends to keep the draw produced by the exhaust fan constant, especially when electronic feedback is used to help stabilize fan speed. However even these variable speed drives are not fully satisfactory at achieving exceedingly uniform flow rates at the inlet to the RTO equipment.
Precise control of the inlet flow rate in RTOs is important in certain applications. For example, in a paint booth or oven, the amount of paint on a part may be altered by changes in the rate at which solvent-laden air is drawn into the RTO. Also, on a conveyor line operation for coating aluminum beverage cans, abrupt pulsations in air pressure at the inlet of an RTO, due to flow reversals within the RTO's heat exchange chamber, may produce a surge of air capable knocking over the empty cans on the conveyor belt. As a final example, solvent-laden air may be "burped" out of the front end of a continuous oven from which contaminated air is being drawn by an RTO, if the RTO unit temporarily but significantly diminishes the rate at which contaminated air is being drawn into the RTO, sending the contaminated air into the plant. These and other kinds of air pressure variation problems still occur with existing RTO equipment.
Another problem with existing RTO equipment is that its operation is subject to variation and change over time. The present inventor has traced the cause of many of these variations to changes in the ambient temperature of the environment in which RTO equipment operates. Those in the art will appreciate that RTO equipment, due to its size, is frequently located outside of a processing plant. Such RTO equipment is thus subject to wide swings in ambient temperature due to climatic changes such as differences between daytime and evening temperatures, winds, rain and seasonal temperature variations. These temperature variations, Applicant has discovered, can adversely affect how consistently the RTO equipment operates. Such time and temperature variations degrades equipment performance, particularly the uniformity of valve operation, which, in turn, produces variations in the inlet flow rate or draw of the RTO equipment.
Accordingly, it is a first important object of the present invention to provide an RTO apparatus that produces a highly stabilized draw of air into the RTO apparatus, even as the inlet valves, outlet valves and flushing valves of the RTO equipment open and close. A related object of the present invention is to provide an improved method for operating a regenerative thermal oxidizer that minimizes changes to mass air flow rates, in spite of the periodic changes to the direction of air flow through each of the heat exchange chambers. In other words, the object is to provide for minimum disturbance of the mass flow rate into the RTO even when each of the heat exchangers are alternately operated in a heat absorbing mode, a heat radiating mode, and a flushing mode. Still another related object is to provide an improved six-step sequence of operation for the inlet, outlet and flushing valves which preserves maximum thermal and emission efficiencies, while achieving very stable flow rates.
A second important object of the present invention is to provide for an improved hydraulic control system that provides for highly stable valve operating characteristics over time, even when the RTO equipment is operated out-of-doors in an environment experiencing wide swings in temperature between day and night or from season-to-season.
One more object of the present invention is to eliminate the problem associated with regenerative thermal oxidizers which sometimes emit solvent-laden air into a plant's environment as inlet, outlet and flushing valves switch from one state to the next.
Still further, it is an object of the present invention to provide an improved RTO apparatus and method for removing the build-up of solid residue materials (i.e., contaminants) from the duct work and valves of the inlet flow structure leading to the RTO's heat exchange chambers. A related object is to shorten bake-out cycle times and reduce fuel consumption by eliminating sources of inefficiencies which arise during such bake-out cycles.